1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to an improvement of focusing characteristics of a color cathode ray tube which has an inner surface of a panel thereof flattened.
2. Description of the Related Art
In a color cathode ray tube used in a television receiver or a personal computer or the like, a panel which forms a screen has inner and outer surfaces thereof formed in a curved surface.
FIG. 6 is a cross-sectional view of a schematic structure of a conventional color cathode ray tube. The color cathode ray tube includes an evacuated envelope which is constituted of a panel 1 which forms a phosphor screen on an inner surface thereof, a neck 3 which houses an electron gun and a funnel 2 which connects the panel 1 and the neck 3.
Both inner and outer surfaces of the panel 1 have the radii of curvature of approximately 1.35xc3x97103 mm. A shadow mask (omitted from the drawing) which constitutes a color selection electrode is arranged close to the screen formed on the inner surface of the panel 1. In the drawing, Rix indicates an equivalent radius of curvature in the horizontal direction of the inner surface of the panel 1 and Riy indicates an equivalent radius of curvature in the vertical direction of the inner surface of the panel 1.
An electron gun 10 which irradiates three electron beams in the direction toward the screen is housed in the inside of the neck 3. A deflection yoke 11 for deflecting these three electron beams irradiated from the electron gun 10 in the X direction (horizontal direction) and the Y direction (vertical direction) of the screen is mounted on a transitional region between the funnel 2 and the neck 3 of the evacuated envelope. Here, Z-Z indicates a tube axis of the color cathode ray tube.
The contour of the panel 1 is of an approximately rectangular shape having a long axis in the X direction and a short axis in the Y direction. A display region AR is formed on a front surface of the panel 1. The electron beams irradiated from the electron gun 10 are deflected from the center of the deflection magnetic field formed by a deflection yoke (deflection center) 12 by an effective maximum deflection angle xcex8.
The focusing characteristics of the electron beams on the screen exert the influence to the quality of a reproduced image. The electron gun 10 includes a cathode, an electron beam generating part made of a first electrode and a second electrode which are arranged in sequence from the cathode to the screen side and a focusing electrode and an anode electrode which are arranged in sequence at the screen side of the electron beam generating part.
As parameters which determine the focusing characteristics, the electrode length Lm of the electron gun 10, the distance Lc between the electron gun 10 and the screen, the distance (Lw/cos xcex8) between the deflection center and an outermost point of the effective display region and the distance Ld between an anode-side end portion of the focusing electrode and the deflection center 12 are considered.
Here, it is necessary to set the distance Ld to not less than a given distance which can prevent the electron gun 10 from being influenced by the deflection magnetic field. Further, the longer the electrode length Lm of the electron gun 10, the focusing characteristics is improved.
Recently, in the field of color cathode ray tubes also, so-called flat-face type or flat-panel type color cathode ray tubes which have outer surfaces of panels thereof approximately flattened have been widely adopted.
As a prior art which relates to this flat-panel type (hereinafter also called flat-face panel type) color cathode ray tube, for example, Japanese Laid-open Patent Publication 64451/1998 can be named.
With respect to the color cathode ray tube which has the inner and outer surfaces of the panel flattened, the further enhancement of the focusing characteristics has been requested. This is because that by flattening the inner surface of the panel, an incident angle of the electron beam on the panel becomes large at a peripheral portion of the screen and hence, the focusing characteristics are deteriorated.
Further, with respect to a television receiver or an equipment such as an information terminal of a personal computer or the like, in addition to the demand for the enlargement of the screen size (magnitude of the screen), the shortening of the depth size has been demanded. Particularly, the color cathode ray tube whose maximum deflection angle of electron beam becomes 90xc2x0 has an elongated funnel portion and hence, it is difficult to shorten the depth size of the color cathode ray tube. Further, it is also difficult to shorten the distance between the inner surface of the panel and the screen-side end face of the focusing electrode. Still further, it is also difficult to elongate the electrode length of the electron gun.
So long as the electrode length of the electron gun is held at a large value, the total length of the color electron gun becomes long and hence, it is difficult to satisfy the demand for the shortening of the depth size.
The distance Lc between the electron gun and the screen can be expressed by a following equation when the equivalent radii of curvature in the X direction and in the Y direction of the inner surface of the panel are set to Rix and Riy.
Lc=Lw+Ld+Rixxe2x88x92{square root over ( )}((Rixxe2x88x92Riy+{square root over ( )}(Riy2xe2x88x921502))2xe2x88x922002)
In the present invention, by making the equivalent radii Rix, Riy in the X direction and in the Y direction of the inner surface of the panel large, the distance Lc between the electron gun and the screen is made small and the electrode length Lm of the electron gun is extended.
The present invention provides a flat-panel type color cathode ray tube which can overcome the above-mentioned drawbacks of prior art and can shorten the entire length while ensuring the favorable focusing characteristics.